Method for producing a metal coated heavy metal foam

Abstract

 A method for producing a metal foam comprises the steps of providing an open cell core metal foam comprising a first metal or alloy; and coating the core metal foam with a second metal or alloy, which is applied onto the core metal foam in a liquid state. The first metal or alloy has a melting point that is higher than that of the second metal or alloy.

Description

FIELD OF THE INVENTION

[0001] The present invention generally relates to the production of metal foams, in particular of heavy metal foams.

BACKGROUND OF THE INVENTION

[0002] As a consequence of the requirements regarding the conservation of energy, miniaturisation and the need for applying new design concepts in many fields of engineering, including appliances (air-conditioning, refrigerators), transportation (car radiators, air-conditioning), general industry (heating, cooling) and power electronics (cooling), heat-exchange devices will be asked to perform the same function more efficiently and within an increasingly smaller volume. This means a significant increase in the area-dependent heat dissipation requirements. Managing this heat dissipation, to extend the life of a power transistor, for example, becomes a challenge given the even greater restrictions on the space that is allocated for the heat exchanger. New ideas are constantly being sought to further promote the miniaturisation of the entire electronic package including the electronic components and the heat exchanger.

[0003] In designing an optimal heat exchanger, many factors of the heat exchanger must be considered. These include the processability, cost and thermal conductivity of the material, the overall required size of the heat exchanger for a given function and thermal efficiency, the resistance of the material to mechanical damage (through solicitations resulting from forming/joining operations and in-service vibration and thermal expansion), to corrosion (including high temperature-induced corrosion), and the available convection and conduction surface areas. For each design, a balance of these and other factors must be reached within a specific cost consideration. Quite often, one of the greatest cost influencing factors is the material itself, which frequently limits the design of the heat exchanger to conventional materials, but with well engineered configurations.

[0004] An open cell metal foam fits this description well. The open cell geometry of the foam provides a labyrinth of fluid flow passages while having a large convection surface area for enhanced heat exchange between the solid and fluid phases. A particularly appreciated material that meets all these requirements is an open cell aluminium foam.

[0005] Many different processes for manufacturing metal foams have been developed, and can be distinguished according to the state of matter in which the metal is processed: liquid, solid or ionised.

[0006] Processes starting from liquid metal are generally called melt metallurgical processes. Such a typical process is described in DE 43 26 982, wherein cast bodies of metal foam are produced. A melt, e.g. of aluminium, is held in a liquid state in a vessel provided with an agitator, which generates a foam from the melt metal. The foam is then forced into a cavity to cast it into a predetermined shape.

[0007] Processes involving solid matter processing are based on powder metallurgy. As described in DE 41 01 630, a powder metallurgical metal foam can be produced from a metal powder and a foaming agent which releases gas upon heating. A mixture of the foaming agent and metal powder is hot compacted and shaped to a compact of metal particles which are held firmly together to provide a matrix in which the expanding agent particles are held in a gas-tight manner. The compacts are introduced into a heated steel mould and are foamed by heating, with the metal foam expanding to fill the mould cavity.

[0008] These techniques allow the manufacture of relatively heavy metal foams, i.e. having thick walled cells. Unfortunately, these heavy metal foams have a mainly closed cell structure, so that they have a low capacity for thermal exchange with a surrounding medium, despite their heavy weight which allows for a good heat-transfer within the solid foam itself. Processes that yield open-pore material or that remodel the structure of a foam featuring an appreciable number of closed cells are not readily available for metal foams. Furthermore, with known techniques it is difficult to achieve reproducible manufacturing conditions and to control the foam porosity.

[0009] Metal foams can also be manufactured by means of electrolytic techniques (ionised metal processing). In a typical electrolytic process, a porous support such as a polymer foam is rendered electrically conductive, and this porous support is then passed through an electroplating bath to deposit a desired thickness of metal on the porous support. If desired, the initial porous substrate can then be removed by a thermal treatment. Such a process is much more reproducible, as the characteristics of the polymer foam are reproduced rather closely, and the wall thickness of the cells can precisely be controlled during electrolysis. In this connection, particularly interesting polymer foams are "reticulated" polyurethane foams, since they have very regular and highly porous structures. Electrodepositing a metal, e.g. copper, on such a polyurethane foam thus leads to an open cell metal foam having a highly reproducible structure, and very good thermal conductivity.

[0010] However, the process is not adapted for producing heavy metal foams, because electrolysis is a relatively slow process if it comes to depositing large metal quantities. In addition, it does not allow the manufacture of aluminium foams, since aluminium cannot be deposited from an aqueous electrolyte.

[0011] Hence, there is a need for an alternative method for producing a metal foam that is particularly adapted for producing heavy metal foams.

SUMMARY OF THE INVENTION

[0012] According to the present invention, a method for manufacturing a metal foam comprises the steps of:

providing an open cell core metal foam comprising a first metal or alloy; and

coating said core metal foam with a second metal or alloy, which is applied onto said core metal foam in a liquid state.

[0013] It will be appreciated that the first metal or alloy has a melting point that is higher than that of the second metal or alloy. As a result, an open cell metal foam comprising a core metal foam of the first metal or alloy substantially coated with a layer of the second metal or alloy is obtained.

[0014] Applying the second metal or alloy in liquid state onto the core metal foam permits the formation of a thick coating in a short lapse of time, and is namely much faster than electroless or electrolytic techniques. The liquid metal penetrates to the heart of the core metal foam, and permits a homogeneous increase of cell wall thickness. The application of the second metal in liquid state is controlled in such a way as to keep the open cell structure of the core metal foam (minimise cell filling). Hence, the present method is particularly well suited for the production of open cell heavy metal foams, namely for use in heat exchanger applications. The use of a core metal foam as precursor permits to control the characteristics of the heavy metal foam to be produced, which also leads to good reproducibility.

[0015] It will be understood that the core metal foam plays the role of an inner supporting structure, which firstly can be contacted with the liquid second metal without risk of re-melting (due to the difference in melting point). Secondly, the presence of the core metal foam is extremely interesting in that it allows welding or brazing of the metal coating (i.e. second metal or alloy), without any risk of collapse of the metal foam. Indeed, since the core has a higher melting point, it will not melt when subjected to temperatures about the melting temperature of the second metal or alloy.

[0016] The term "core metal foam" herein means any metallic open cell structures, not only truly cellular material. Porosity of the core metal foam is a prerequisite for the present method, since liquid metal has to penetrate in the foam via its pores to increase the cell wall thickness. Therefore, the core metal foams to be used in the present method will have a mainly open cell structure (i.e. having a majority of open cells).

[0017] In a preferred embodiment, the step of coating the core metal foam with the second metal or alloy is carried out by hot-dip coating. The core metal foam is dipped in a liquid metal bath comprising the second metal or alloy. This embodiment proves particularly easy and cheap to implement, since hot-dipping is a well controlled, robust and cost-effective industrial process.

[0018] Advantageously, the coating step is preceded by a pre-treatment step, wherein the core metal foam is subjected to a treatment to enhance the wettability and/or to prevent oxidation of the first metal or alloy. This can be done by applying a conventional flux on the core metal foam. However, the oxidation of the core metal foam before coating is preferably prevented by preheating the metal foam in a protective atmosphere, e.g. a neutral or reducing atmosphere, and guiding the metal foam from the protective atmosphere directly into the liquid metal bath. If needed, the pre-treatment step may also comprise cleaning/deoxidising of the core metal foam.

[0019] To control, i.e. avoid or strongly reduce, the diffusion between the liquid metal or alloy in the liquid metal bath and the core metal foam, a diffusion barrier layer may advantageously be applied onto the core metal foam before the coating step.

[0020] After coating of the core metal foam with the second metal or alloy, the metal foam is advantageously subjected to a wiping step, in order to remove excess liquid metal. This allows controlling the thickness of the applied layer of second metal or alloy, before it completely solidifies. A variety of known techniques are applicable for wiping the excess metal, such as gas wiping (e.g. with air or N₂), electromagnetic wiping or mechanical wiping (e.g. ultrasonic wiping).

[0021] The present method shall advantageously comprise a further passivation step to prevent corrosion of the second metal or alloy.

[0022] The core metal foam may take various forms. Preferably, the core metal foam is in strip form, and is continuously coated with the second metal or alloy that is applied in liquid state. In the case of hot-dip coating, the strip of core metal foam is continuously passed through the liquid metal bath comprising the second metal or alloy. It is clear that when a strip of foam is to be processed in accordance with the present method, the optional pre-treatment, wiping and passivation steps are also preferably carried out continuously.

[0023] In another embodiment, the core metal foam is in the form of a coiled strip, and is dipped into a liquid metal bath comprising the second metal or alloy. In the hot-dip coating bath, the liquid metal penetrates in the coil of core metal foam, and when the coil is removed from the bath and the molten second metal solidifies, it holds together the layers in the coil, so that a cylindrical body of metal foam is obtained.

[0024] In a further embodiment of the present method, a number of pieces of core metal foams having predetermined shapes are subjected to the coating step. The obtained pieces of metal foam are then assembled to be in contact with one another and this assembly of pieces of metal foam is subjected to heating in a furnace. In the furnace, the coating of second metal of each piece partially melts, which leads, after solidification, to the welding of adjacent pieces of metal foam so that a larger body of metal foam is obtained. Alternatively, solder material (having a melting point below the melting point of the first and second metals) can be inserted between the pieces of core material, and the assembly then heated to a temperature corresponding to the melting temperature of the solder material.

[0025] In a further alternative embodiment, a number of sheets of core metal foam having predetermined shapes are piled up with sheets of the second metal or alloy, in an alternating manner. This assembly is then heated in a furnace in which temperature and atmosphere are adjusted in such a way that the sheet of second metal or alloy melts and spreads into the sheets of core metal foam, the temperature of the furnace being however below the melting temperature of the first metal foam. In such an embodiment, the core metal foam is preferably made of a metal that is highly wettable, or covered by a diffusion barrier layer of highly wettable metal.

[0026] Metal foams of a variety of metals can be manufactured with the present method, depending on the foreseen application. Hence, the first and second metals or alloys can be made of a variety of metals, as long as the difference in melting point is respected. For ease of implementation and use, the difference in melting point between the first metal and second metals (resp. alloys) is preferably of at least 60°C.

[0027] Preferred metals for the core metal foam are copper, copper alloys, nickel or nickel alloys. For the production of aluminium foam, the second metal is aluminium or an aluminium alloy.

[0028] As already mentioned, the present method permits the manufacture of heavy metal foams that are particularly suited for use in heat exchanging applications. For such applications, the metal foams shall preferably have a porosity in the range of 60 to 90% with 30 to 100 pores per inch (ppi). The measurement of porosity is linear; it corresponds to the number of unit cells (pores) that are counted along a line of one inch in length.

[0029] For the manufacture of aluminium foam according to a preferred embodiment of the present method, the core metal foam preferably consists of copper coated by a diffusion barrier layer of nickel,and the second metal or alloy is aluminium or an aluminium alloy. The core metal foam may have a thickness about 2.0mm and a specific weight of copper in the range of 150 to 1 000 g/m². The operating conditions―in particular the coating and wiping steps―are preferably controlled in such a way as to form an aluminium coating layer (second metal) having a specific weight of aluminium in the range of 1 000 to 2 000 g/m². Alternatively, a very light nickel foam (preferably 2 mm thick and having a specific weight below 300 g/m²) can advantageously be used as core metal foam (instead of a copper foam) to produce aluminium foams.

[0030] It will be noted that the weight of each metal in the foams are expressed herein with reference to their "specific weight". This term is herein understood as the mass of metal per apparent unit area of the metal foam. For example, if the specific weight of a given metal, e.g. copper, in a flat metal foam strip is 1 500 g/m², it means that a piece of this metal foam strip having apparent external dimensions of 1 m x 2 m will contain 3 000 g of copper.

[0031] The core metal foam is preferably produced by electroplating a first metal or alloy onto a porous substrate having an electrically conductive surface. Such a porous substrate can consist of a variety of materials such as polymeric foams, carbon or graphite foams, silicate foams and other organic or inorganic open-cellular materials; the electrical conductivity of such substrates can be increased if needed by means of conventional techniques. This porous initial structure may later be eliminated.

[0032] Preferably, the porous substrate is a polymer foam, more preferably a reticulated polyurethane foam, having a surface which is rendered electrically conductive by a chemical coating/treatment or vacuum metallization. Core metal foams prepared by electrolysis onto a polymer foam have reproducible characteristics and can easily be produced as coiled strips.

[0033] It will be understood that it is interesting to use an electroplated foam with thin walled cells as core metal foam, as it minimises the time for producing the precursor material. However, the core metal foam should preferably exhibit sufficient ductility and tear resistance to permit its handling, namely in the form of strips and coils.

[0034] Regarding the thickness of the core metal foam, a great range of thickness can be used―typically from a few mm to several centimetres and more-as long as a proper distribution of the liquid second metal in the cellular structure of the core metal foam can be ensured. When the core metal foam is prepared by electroplating, the porous substrate (e.g. polymer foam) on which the first metal is plated preferably has a thickness in the range of 1 to 5 mm, which permits a good compromise between plating homogeneity and productivity.

[0035] According to another aspect of the present invention, a metal foam comprises an open cell core metal foam of a first metal or alloy and a coating of a second metal or alloy substantially covering the core metal foam. The first metal or alloy has a higher melting point than the second metal or alloy. The second metal or alloy can be applied onto the core metal foam by different methods. However, the second metal or alloy is preferably applied in a liquid state, so that the core metal foam is coated with a solidified melt of the second metal or alloy. Most preferably, the coating is applied by hot-dip coating in a liquid metal bath comprising the second metal or alloy.

[0036] In the present metal foam, the cell wall thickness ratio between the second metal or alloy and the first metal or alloy is preferably between 3 and 50.

[0037] It will be noted that the present metal foam is easy to use and to assemble to other metallic pieces. It can be heated―locally or generally―at temperatures up to the melting point of the second metal or alloy, without risk of collapse of the metal foam, since the core metal foam has a higher melting point. The present metal foam is thus particularly well adapted to be joined by brazing or welding to a variety of metallic pieces, but also to similar foams. For example, in the latter case, two or more pieces of the present metallic foam can be joined to form a larger body of metallic foam by putting them in contact with each other (possibly in presence of a solder in massive or powdered form) and heating them so as to melt the second metal coating and weld (or solder) these pieces together.

[0038] The metal foam of the invention can be used for a variety of applications. In particular, the present foam can advantageously be used as part of a heat exchanger, since the open cell geometry of the foam provides a labyrinth of fluid flow passages while having a large convection surface area for enhanced heat-exchange between the solid and fluid phases.

[0039] The metal foam preferably has a porosity in the range of 60 to 90 %, with 30 to 100 pores per inch. This porosity is considered as particularly adapted for heat exchange applications.

[0040] The first metal is preferably copper, copper alloy, nickel or nickel alloy. The second metal is preferably aluminium or aluminium alloy.

[0041] To control the diffusion between the first and second metals, the core metal foam is preferably coated by a diffusion barrier layer. When using a copper based core metal foam, a particularly preferred diffusion barrier is a nickel plating substantially covering the core copper foam.

[0042] In a preferred embodiment, in particular for heat exchange applications, the first metal or alloy is copper and a specific weight of the first metal in the core metal foam is preferably in the range of 150 to 1 000 g/m² (for a strip thickness of typically 2.0 mm); and the second metal or alloy is aluminium or aluminium alloy with a specific weight preferably in the range of 1 000 to 2 000 g/m².

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG.1: is a scanning electron microscope view of the structure of a core metal foam;

FIG.2: is a schematic illustration of a production line for implementing a preferred embodiment of the present method;

FIG.3: is a sketch of the structure of a metal foam produced in accordance with the present method; and

FIG.4: is a sketch illustrating an assembly for manufacturing a metal foam in accordance with another embodiment of the present method.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0044] A preferred embodiment of the present method will now be described in more detail, wherein an aluminium foam is produced by coating a core metal foam made of copper in an aluminium-based hot-dip coating bath.

A. Preparation of the core metal foam

[0045] The core metal foam is preferably prepared by an electrolytic technique. This can be done by electrodepositing, in an aqueous electrolytic bath, a first metal or alloy on a non-metallic foam or porous substrate. A prerequisite for this method is that the porous substrate has a sufficient electrical conductivity for electrodeposition. If required, various techniques can be used to increase the substrate conductivity among which chemical treatment or vacuum deposition. The electrically conductive porous structure is then subjected to electro-deposition to form thereon a layer of the desired metal to a certain thickness. More preferably, the core metal foam is produced by electro-deposition in a cylindrical electrolytic cell, as described in WO 02/22914. The initial porous substrate can generally be removed by thermal treatment.

[0046] The initial foam material preferably is a reticulated polyurethane foam, which has a fine and regular open cell structure. Such a polyurethane foam can be easily produced and processed in strip foam.

[0047] In the present embodiment, the polyurethane foam is firstly pre-metallized by vacuum deposition. A very thin layer of copper is formed, preferably in an amount of e.g. 3 to 10 g/m² on a 2 mm thick strip of polyurethane foam. The premetallized polymer foam is then passed through a cylindrical electroplating cell to deposit a desired thickness of copper, thereby obtaining the core metal foam. The electroplating parameters are preferably adjusted to form a core metal foam with a specific weight of copper in the range of 150 to 1 000 g/m².

[0048] For productivity reasons, the initial polyurethane foam is preferably a coiled strip of polyurethane foam, which can be subjected to continuous pre-metallizing and electroplating.

Example:

[0049] Fig.1 shows the structure of such a copper foam obtained by copper electroplating onto a reticulated polyurethane foam. Such a highly porous metal foam is also referred to as "lattice block material" (LBM). LBMs are repeated cells consisting of straight struts each connecting two nodes, which form a perfectly regular cell structure. Structural details about such a core copper foam based on reticulated polyurethane are typically the following:

• cell structure: closely resembling a perfect dodecahedron (12 pentagon-shape windows and 30 struts);
• cell diameter: approximately ranging from 2700 µm (at 20 ppi) to below 500 µm (at 100 ppi);
• cell volume: typically about 0.2 mm³ at 60 ppi;
• porosity: typically above 95%.
• number of cells: an estimated 2·10⁷ (at 10ppi) to 2.10¹⁰ (at 100ppi) cells per cubic metre.

B. Production of the metal foam

[0050] After preparation of the core metal foam, the latter is then subjected to a coating step, wherein liquid aluminium or aluminium alloy is applied preferably by hot-dip coating. A production line 10 for carrying out the coating step is shown in Fig.1.

[0051] In Fig.1, reference sign 12 indicates a coiled strip of core copper foam, produced as described above. The strip of core copper foam 12 is unwound and guided to a pre-treatment furnace 14 via a guide roll 16. The furnace comprises a hot zone 18 provided with heating means (not shown) that are preferably adapted to create a temperature of about 400°C to 700°C therein, adjustable with line speed. The strip of core copper foam 12 is introduced into the furnace through an inlet 20, and thus directly penetrates into the hot zone 18. Advantageously, a neutral or reducing atmosphere is maintained in the furnace 14 to reduce surface oxides that may be present and/or to avoid oxidation of the core copper foam while preheating the latter. The atmosphere in the furnace 14 is preferably a reducing atmosphere comprising 3%-100% hydrogen (remaining gas being nitrogen).

[0052] Following the hot zone 18, the pre-heated strip of core copper foam 12 travels through an intermediate zone 22, in which the temperature of the core copper foam 12 is optionally adjusted for the hot-dipping. The strip of core copper foam 12 is then introduced through a snout 24 into a liquid metal bath 26 without making any contact with air. The liquid metal bath 26 comprises molten aluminium or aluminium alloy.

[0053] In the liquid bath 26, the molten aluminium penetrates through the pores of the copper foam 12 deep into the foam, so that molten aluminium is applied all over the surface of the copper foam 12. The core copper foam 12 plays a role of inner supporting structure, that can be contacted with the liquid aluminium without risk of melting, due to the difference in melting point. Indeed, the melting point of copper is 1083°C and the working temperature of aluminium or common commercial aluminium alloys is between 585°C and 700°C.

[0054] As can be seen in Fig.1, the strip of copper foam 12 is turned upwardly around a pot roll 28 in the liquid metal bath 26, so that it leaves the bath 26 through the upper surface. As the obtained aluminium foam (resulting from the coating of the copper foam 12) is pulled out of the liquid bath 26, a certain amount of aluminium coating will solidify around the copper struts, thereby strengthening the original copper skeleton.

[0055] It will be noted that in order to prevent problems of diffusion between the core copper foam and the liquid aluminium bath, in particular when using an aluminium-silicon alloy, the contact between the liquid metal and the core metal foam should preferably be as short as possible (if possible less than 1 second). These problems of diffusion can advantageously be controlled by applying to the core copper foam prior to the dipping step a diffusion barrier layer such as a nickel plating. The diffusion barrier layer can e.g. be applied onto the core copper foam after removal of the polyurethane foam substrate by heat-treatment, also by electroplating. It will be further noted that a nickel foam could alternatively be used as core metal foam, since it does would not need a barrier layer before dipping in a silicon containing aluminium alloy. A core nickel foam can also be produced by electroplating a reticulated polyurethane foam as described hereabove, or by other known processes.

[0056] Reference sign 30 indicates gas wiping means, surrounding the produced strip of aluminium foam 32 exiting the bath 26. The gas wiping means 30 blows air or a protective gas (nitrogen), optionally preheated, towards both sides of the strip of aluminium foam 32 for wiping off excess liquid aluminium and thus controlling the thickness of the aluminium coating layer. The air knives will be adapted to the specific task of wiping a porous strip; preferably, they will be positioned at varying height so as not to interfere. The obtained open cell aluminium foam 32 is then allowed to cool down and will be coiled.

[0057] If desired, the wiping step can be followed by a passivation step (not shown in Fig.1), in order to apply a corrosion-protection layer onto the aluminium foam 32.

[0058] Although not implemented in the present embodiment, the furnace 14 may comprise a bottom surface that is inclined towards the liquid bath 26, and on which the core strip of foam is made to slide. This permits to take advantage of the effect of gravity to cause the progression of the core metal foam in the furnace 14. The driving force exerted by driving rolls can thus be reduced, thereby reducing the tear and stress to which the core metal foam is subjected.

[0059] For production of aluminium foams, the liquid metal bath preferably consists of pure aluminium, which has the highest coefficient of thermal conductivity. However, the liquid metal bath may also advantageously comprise an aluminium alloy, namely an alloy comprising up to 13 wt.% silicon and e.g. about 500 ppm of strontium, for globularising the silicon needles. Although the use of such an alloy reduces the thermal conductivity, it facilitates the overall processability (coating, welding/soldering). Other common aluminium alloys may be selected, notably in regard to corrosion resistance, such as aluminium-zinc alloys containing a majority of aluminium and up to 45 % zinc.

[0060] Fig.3 schematically shows the structure of the obtained open cell aluminium foam 32. Reference sign 34 indicates two initial copper struts, the plating being not shown. Each strut 34 is coated with a relatively thick aluminium layer 36. In this example, the initial polyurethane substrate has been removed, and the core copper foam thus consists of hollow struts (voids indicated by reference sign 38).

[0061] It will however be understood that, due to handling and other stresses to which the foam may be subjected, some struts 34 may be disconnected from one respective node. In addition, some struts 34 may be partially broken and holes may have formed. It follows that, when dipping the core metal foam into the liquid metal bath, liquid metal may fill the inside of the hollow struts.

[0062] More generally, this means that metal foams manufactured in accordance with the present method may exhibit a core metal structure made of a first metal that is substantially coated with a layer of a second metal, and that the structural members forming the core metal foam may also contain the second metal.

C. Use of the metal foam

[0063] The aluminium foam produced according to the present method is relatively heavy while having a very high porosity, and is particularly well suited to be used as part of a heat exchanger, e.g. in combination with (alumi nium- or copper-) tubing or flat heat-conducting surfaces. Pieces of aluminium foam can be cut from the manufactured strip of aluminium foam, and directly applied to a component to be cooled, e.g. an electric/electronic part. A piece of aluminium foam can easily be welded or soldered to a metallic part since the core copper foam plays the role of supporting structure that prevents the collapse of the aluminium foam when heating the latter at temperatures slightly above the melting point of the aluminium coating.

[0064] It is to be further noted that a piece of aluminium foam produced by the present method can not only be easily welded/soldered to another metallic element by simple heating (eventually following fluxing, or within an adequate protective atmosphere) at a temperature above the melting point of the alumi n-ium coating (or the solder), but also to a similar piece of aluminium foam. For example, large aluminium foam bodies can be formed by cutting a number of pieces of aluminium foam from the strip 32, piling them one above another, and heating the pile in a furnace to a temperature being slightly above the melting point of the aluminium coating or the solder. Each of the pieces of the pile will be welded/soldered to the adjacent pieces of aluminium foam and/or to alumi n-ium piping, for example, thus forming upon cooling a larger body of aluminium foam. Due to the presence of the core copper foam, which has a much higher melting point, the assembly does not collapse. Such a body of aluminium foam can furthermore be cut into pieces of desired shapes.

[0065] It will be further noted that, in order to form a heat-exchanger, piece(s) of core aluminium foam can brought into contact with a heat-carrying element (e.g. a pipe or heat sink) and the liquid second metal applied in such a way that it penetrates into the foam and simultaneously forms a joint at the interface foam/heat-carrying element. For example, one or more strips of foam can be wrapped around a metal pipe, and this assembly then dipped into a liquid bath comprising the second metal or alloy. The second molten metal will thus be applied on both the core metal foam and the pipe, and provide a solid joint between the latter upon solidification of the second metal.

D. Alternative embodiment

[0066] An alternative embodiment of the present method that allows producing of a large body of heavy metal foam is schematically shown in Fig.4. A number of pieces of core metal foam 40 having predetermined shapes (e.g. rectangular) are piled up with sheets 42 of the second metal or alloy, in an alternating manner.

[0067] This assembly is then heated in a furnace in which temperature and atmosphere are adjusted in such a way that the sheet of second metal or alloy melts and spreads into the pieces of core metal foam, the temperature of the furnace being however below the melting temperature of the first metal foam. Hence, the core metal foam remains solid while the sheets of second metal or alloy melt and liquid second alloy distributes in the pieces of core metal foam 40 (as indicated by arrow 44).

[0068] After solidification of this assembly, a large body of metal foam is obtained, which consists of core metal foam pieces united by a coating layer of the second metal or alloy. In such an embodiment, the core metal foam is preferably made of a metal that is highly wettable, or covered by a diffusion barrier layer of highly wettable metal.

Claims

1. A method for producing a metal foam comprising the steps of:

providing an open cell core metal foam comprising a first metal or alloy;

coating said core metal foam with a second metal or alloy, which is applied onto said core metal foam in a liquid state;

wherein said first metal or alloy has a melting point that is higher than that of said second metal or alloy.

2. The method according to claim 1, wherein said coating step comprises dipping said core metal foam in a liquid metal bath comprising said second metal or alloy.

3. The method according to claim 1, wherein said coating step comprises bringing said core metal foam into intimate contact with a liquid film comprising said second metal or alloy.

4. The method according to claim 1, 2 or 3, comprising a pre-treatment step prior to said coating step, wherein said core metal foam is treated in order to enhance the wettability and/or prevent oxidation of said first metal or alloy.

5. The method according to claim 4, wherein a flux is applied on said core metal foam.

6. The method according to claim 4 or 5, wherein said core metal foam is preheated in a protective atmosphere and is guided from said protective atmosphere directly into said liquid metal bath.

7. The method according to any one of claims 4 to 6, wherein said pre-treatment step comprises cleaning/deoxidising the core metal foam.

8. The method according to any one of the preceding claims, wherein a diffusion barrier layer is applied onto said core metal foam before said coating step.

9. The method according to any one of the preceding claims, comprising the step of wiping off excess liquid metal after said coating step.

10. The method according to claim 9, wherein said wiping step involves gas wiping, electromagnetic wiping or mechanical wiping.

11. The method according to claim 9 or 10, comprising a further passivation step following said wiping step.

12. The method according to any one of the preceding claims, wherein said first metal or alloy is nickel, a nickel alloy, copper or a copper alloy.

13. The method according to any one of the preceding claims, wherein said second metal or alloy is aluminium or an aluminium alloy.

14. The method according to any one of the preceding claims, wherein said core metal foam is in strip form, and is continuously coated with said second metal or alloy.

15. The method according to any one of claims 1 to 13, wherein said core metal foam is a coiled strip and is dipped into a liquid metal bath comprising said second metal or alloy to form a cylindrical body of metal foam.

16. The method according to any one of claims 1 to 13, wherein
a number of sheets of core metal foam having predetermined shapes are piled up with sheets of the second metal or alloy, in an alternating manner, so that each piece of core metal foam is between two sheets of the second metal or alloy;
the obtained assembly is heated in a controlled atmosphere furnace at a temperature above the melting temperature of said second metal or alloy but below the melting temperature of said first metal or alloy, so that said second metal spreads into said sheets of core metal foam; and
the assembly is allowed to cool.

17. The method according to any one of claims 1 to 13, wherein
a number of pieces of core metal foam having predetermined shapes are subjected to said coating step;
the obtained pieces of metal foam are then assembled to be in contact with one another;
and the assembly of pieces of metal foam is subjected to heating in a furnace so as to form a body of metal foam due to welding of the coating layers of said second metal or alloy.

18. The method according to any one of the preceding claims, wherein said core metal foam is produced by:

providing a porous substrate having an electrically conductive surface;

electroplating said first metal or alloy onto said porous substrate.

19. The method according to claim 18, wherein said porous substrate is a reticulated polyurethane foam having a surface, which is rendered electrically conductive by chemical treatment or vacuum metallization.

20. A metal foam comprising
an open cell core metal foam comprising a first metal or alloy;
a coating comprising a second metal or alloy substantially covering said core metal foam;
wherein said first metal or alloy has a higher melting point than said second metal or alloy.

21. The metal foam according to claim 20, wherein said second metal or alloy is a solidified molten metal.

22. The metal foam according to claim 20 or 21, wherein said metal foam has a porosity in the range of 60 to 90 %, with 30 to 100 pores per inch.

23. The metal foam according any one of claims 20 to 22, wherein said first metal or alloy is copper, copper alloy, nickel or nickel alloy.

24. The metal foam according any one of claims 20 to 22, wherein a barrier diffusion layer covers said core metal foam.

25. The metal foam according to any one of claims 20 to 24, wherein said second metal or alloy is aluminium or an aluminium alloy.

26. The metal foam according to any one of the claims 20 to 25, wherein the cell wall thickness ratio between said second metal or alloy and said first metal or alloy is between 3 and 50.

27. The metal foam according to any one of claims 20 to 26, wherein
said first metal or alloy is copper or a copper alloy and said core metal foam has a mass surface density in the range of 150 to 1 000 g/m²; and
said second metal or alloy is aluminium or aluminium alloy formed with mass surface density in the range of 1 000 to 2 000 g/m².

28. Use of a metal foam as claimed in claims 20 to 27 or as produced by the method according to any one of claims 1 to 19, as a heat exchanger.

29. A heat exchanger comprising a metal foam according to any one of claims 20 to 27 or as produced by the method according to any one of claims 1 to 19.

Drawing